The present invention relates to a liquid crystal display using a liquid crystal material having spontaneous polarization and a method of manufacturing the same.
A liquid crystal display has characteristic features such as low consumption power, light weight, and small height and is therefore extensively used as the monitor display of a personal computer, a car navigation system, or the like. However, when compared to a CRT this liquid crystal display has disadvantages such as a low response speed and a narrow viewing angle. As the size and resolution of a liquid crystal display increase, demands for a high response speed and a wide viewing angle are increasing.
A liquid crystal display using a liquid crystal having spontaneous polarization has attracted attention as a display mode capable of high-speed response. Normally, spontaneous polarization is exploited as an intrinsic characteristic or induced by application of an electric field. Examples of a liquid crystal material like this are a ferroelectric liquid crystal (including an SS-FLC (Surface Stabilized Ferroelectric Liquid Crystal), a surface stabilized monostable ferroelectric liquid crystal, a DHF (Deformed Helix Ferroelectric liquid crystal), a twisted FLC (Ferroelectric Liquid Crystal), an APD (Alternating Polarization Domain), and a polymer stabilized ferroelectric liquid crystal), an anti-ferroelectric liquid crystal (including a thresholdless anti-ferroelectric liquid crystal), and a liquid crystal having an electro-clinic effect.
The operation principle of the aforementioned display mode is to transmit/intercept light by using the property that the optical axis of a liquid crystal, which is sandwiched between two polarizing plates whose transmission axes are shifted 90xc2x0 and which can be regarded as a uniaxial crystal, rotate in the plane parallel to the substrate to which the polarizing plate is attached. In this display mode, inconvenience such as a color change occurs when the liquid crystal display is viewed obliquely. This is so because the apparent liquid crystal refractive index when a liquid crystal display is observed in a direction parallel to the optical axis of a liquid crystal differs from that when the display is viewed in a direction perpendicular to the optical axis.
To solve this inconvenience, Jpn. Pat. Appln. KOKAI Publication No. 9-120065 has proposed a method which increases the viewing angle by forming two regions where alignment directions are perpendicular to each other.
In a liquid crystal display described in Jpn. Pat. Appln. KOKAI Publication No. 9-120065, the area of two regions where alignment directions are perpendicular to each other is as small as about the pixel size which is generally a few tens of xcexcm to several hundred xcexcm. Mask rubbing is used to make the alignment directions of these regions different from each other. Mask rubbing is performed by steps of 1) coating a rubbed alignment film with a photoresist, 2) pre-baking, 3) mask exposure, 4) development, 5), post-baking 6) rubbing, 7) photoresist removal, and 8) alignment film cleaning. Mask rubbing thus having complicated steps increases the manufacturing cost, and this lowers the yield.
Jpn. Pat. Appln. KOKAI Publication No. 9-120065 has also proposed a method of forming the two regions only by making the alignment directions of upper and lower substrates perpendicular to each other. Although the steps of this method are simple, the method is inapplicable to a liquid crystal having a nematic phase or an isotropic phase on the high-temperature side of a chiral smectic C phase. This is because a liquid crystal having this phase sequence has no smectic A phase and hence is set in a twisted state when the alignment directions of upper and lower substrates are perpendicular to each other, so it is impossible to form two regions having different smectic layer directions.
A method of forming two regions having different smectic layer directions by using a liquid crystal having a nematic phase or an isotropic phase on the high-temperature side of a chiral smectic C phase is disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 10-221718. In this method, a liquid crystal is filled into a liquid crystal cell having an alignment film under heating and phase-transited by cooling from a nematic phase or an isotropic phase into a chiral smectic C phase, thereby forming the two regions.
When the method described in this Jpn. Pat. Appln. KOKAI Publication No. 10-221718 is used, it is possible to form two regions having different smectic layer directions by using a liquid crystal having a nematic phase or an isotropic phase on the high-temperature side of a chiral smectic C phase. However, this method has the problem that although the two regions can be formed, locations where these two regions are formed cannot be controlled. In some cases, a desired viewing angle or desired gray scale display cannot be obtained.
It is an object of the present invention to provide a liquid crystal display having a wide viewing angle and a liquid crystal display manufacturing method capable of obtaining a liquid crystal display having a wide viewing angle with simple steps.
To achieve the above object, a liquid crystal display according to the first aspect of the present invention comprises a first substrate, a second substrate opposing the first substrate, a liquid crystal material sandwiched between the first and the second substrate and having spontaneous polarization which has one of a nematic phase and an isotropic phase on a high-temperature side of a chiral smectic C phase, a first region of a portion of the liquid crystal material, and a second region of the liquid crystal material adjacent to the first region, wherein the liquid crystal material is aligned such that the direction of a smectic layer in the liquid crystal material in the first region is different from that in the liquid crystal material in the second region.
Also, a liquid crystal display according to the second aspect of the present invention comprises a first substrate, a second substrate opposing the first substrate, a plurality of pixel electrodes formed on a surface of the first substrate which opposes the second substrate, a pair of comb common electrodes formed on that surface of the second substrate, which opposes the first substrate, and having a plurality of interdigitated teeth, and a liquid crystal material sandwiched between the first and the second substrate and having spontaneous polarization which has one of a nematic phase and an isotropic phase on a high-temperature side of a chiral smectic C phase, wherein a direction of a smectic layer in the liquid crystal material in a first region sandwiched between one of the comb common electrodes and the pixel electrode is different from a direction of a smectic layer in the liquid crystal material in a second region sandwiched between the other of the comb common electrodes and the pixel electrode.
The liquid crystal displays according to the first and second aspects described above are preferably constituted as follows.
An angle the smectic layer in the first region makes with that in the second region is 115 to 155xc2x0
The first substrate has a plurality of pixel regions, the first and the second region are present in each of the plurality of pixel regions, and volumes of the liquid crystal material in the first and the second region are substantially equal.
Letting p be a chiral pitch of the liquid crystal material and d be a distance between the first and the second substrate, d less than p.
A tilt angle of the liquid crystal material is substantially 22.5xc2x0.
The liquid crystal display according to the first aspect is desirably practiced such that the first substrate has a plurality of pixel regions, each of the plurality of pixel regions is divided into two regions correspondingly to the first and the second region, and a direction of a dividing line for dividing each of the plurality of pixel regions into the two regions is substantially parallel to an alignment direction of the liquid crystal material with no voltage applied.
The liquid crystal display according to the second aspect is desirably practiced as follows.
A longitudinal direction of the plurality of teeth of the comb common electrodes is substantially parallel to an alignment direction of the liquid crystal material with no voltage applied.
A pitch of the plurality of teeth of the comb common electrodes is shifted a substantially half period from a pitch of the plurality of pixel electrodes.
A pitch of the plurality of teeth of the comb common electrodes is 500 xcexcm or less.
The liquid crystal display further comprises a plurality of capacitor lines formed on the first substrate so as to oppose a plurality of space portions between the plurality of interdigitated teeth of the comb common electrodes.
A method of manufacturing a liquid crystal display according to the third aspect of the present invention comprises the steps of opposing a first and a second substrate to each other, sandwiching between the first and the second substrate a liquid crystal material having spontaneous polarization which has one of a nematic phase and an isotropic phase on a high-temperature side of a chiral smectic C phase, and forming a first and a second region differing in a direction of a smectic layer in the liquid crystal material, by applying voltages having different polarities to the liquid crystal material in the first and the second region when the liquid crystal material transits from one of the nematic phase and the isotropic phase to the chiral smectic C phase.
The step of opposing the first and the second substrate to each other desirably includes the step of opposing a plurality of pixel electrodes formed on that surface of the first substrate, which opposes the second substrate, to a pair of comb common electrodes formed on that surface of the second substrate, which opposes the first substrate, and having interdigitated teeth, and the step of forming the first and the second region desirably includes the step of forming the first region by applying a voltage of one polarity to the liquid crystal material in a region sandwiched between one of the comb common electrodes and a corresponding one of the plurality of pixel electrodes, and forming the second region by applying a voltage of polarity opposite to the one polarity to a region sandwiched between the other of the comb common electrodes and a corresponding one of the plurality of pixel electrodes.
The step of forming the first and the second region desirably includes the step of making volumes of the first and the second region substantially equal to each other.
The step of forming the first and the second region desirably includes the steps of allowing a direction of a smectic layer in the liquid crystal material in the first region and a direction of a smectic layer in the liquid crystal material in the second region to form an angle of 115 to 155xc2x0.
The step of forming the first and the second region desirably includes the step of performing an alignment treatment for an alignment film in parallel with a longitudinal direction of teeth of the comb common electrode, such that the longitudinal direction of the teeth of the comb common electrodes is substantially parallel to an alignment direction of the liquid crystal material with no voltage applied.
In the present invention, when a liquid crystal material transits from a nematic phase or an isotropic phase to a chiral smectic C phase, a voltage of the one polarity is applied to the liquid crystal material in a first region, and a voltage of polarity opposite to the one polarity is applied to the liquid crystal material in a second region. This forms the first and second regions differing in the direction of smectic layers in the liquid crystal material. Since the two regions having different smectic layer directions are formed by application of voltages having opposite polarities, locations where these two regions are formed are controllable. So, a desired wide viewing angle can be obtained. Also, since locations where the two regions are formed can be controlled only by application of voltages, a liquid crystal display having a wide viewing angle can be obtained with simple steps.
As a practical means for applying opposite polarities to a liquid crystal material, a pair of comb electrodes formed on that surface of a second substrate, which opposes a first substrate, and having interdigitated teeth are used as common electrodes. A voltage of one polarity is applied to one of these comb electrodes, and a voltage of opposite polarity is applied to the other. Consequently, a region sandwiched between one comb electrode and a pixel electrode becomes a first region, and a region sandwiched between the other comb electrode and a pixel electrode becomes a second region.
A liquid crystal material having spontaneous polarization has smectic layers. If the interval between these smectic layers changes due to external force, the changed layer interval cannot be restored even when the external force is removed. Therefore, when the display surface of a liquid crystal display using a liquid crystal material having general spontaneous polarization is pushed with a finger, inferior display called alignment destruction occurs. In the present invention, however, two regions having different smectic layer directions are formed. Accordingly, the propagation of external force which changes layers can be interrupted by the boundary between the first and second regions, thereby preventing alignment destruction.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.